Btrfs: remove field tree_mod_seq_elem from btrfs_fs_info struct

It's not used anywhere, so just drop it.
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ctree.h

@@ -1462,7 +1462,6 @@ struct btrfs_fs_info {
 	spinlock_t tree_mod_seq_lock;
 	atomic64_t tree_mod_seq;
 	struct list_head tree_mod_seq_list;
-	struct seq_list tree_mod_seq_elem;
 
 	/* this protects tree_mod_log */
 	rwlock_t tree_mod_log_lock;